The present invention relates to the art of equalization of audio frequency signals, and more particularly ot a self-correcting audio equalizer.
In conventional high fidelity sound reproduction systems, the chief concern of the user is that the sound reaching the listener should conform as precisely as possible to the supplied source signal, whether it be from a turntable, tuner, tape deck, or other source. The impact of current solid state technology in this field has been such that the electronic components, themselves, add very little coloration to the audio signals which are being processed. The same cannot be said, however, for the final steps in the sound reproduction process. Both the high fidelity speakers which actually generate the acoustics signals and the listening environment in which the signals are propagated significantly influence the fidelity of the reproduced sounds, with the latter being the predominant influence of the two.
The difficulty with the listening environment arises from the difference in its responses to different frequency sounds. Some listening environments may be quite lively, providing multiple reflections of high frequency components, whereas others may be quite dead, providing substantial damping of high frequency components. In either case the frequency versus amplitude characteristics of the reproduced sound will be altered. The nature and extent of the alteration will thus vary from listening environment to listening environment, even if the same electronic and speaker components are employed in all cases.
To combat the influence of the listening environment upon the fidelity of reproduction of the audio signal, it has become popular to introduce modifications in the frequency response characteristics of the audio system which compensate for the colorations introduced by the listening environment. This is generally accomplished by means of an audio equalizer which is interposed in the audio signal path between the signal source and the speakers.
Normally, the equalizer comprises a plurality of contiguous narrow band filters covering the entire audio band. The various output signals from the filters are each attenuated by a respectively controlled amount, and are then added back together to form the equalized signal for application to the power amplifier and speakers. The modification in the frequency versus amplitude characteristic introduced by the equalizer will, of course, depend upon the settings of the various attenuators.
Conventionally, these are manually adjusted in the following manner. A signal having a known frequency distribution (such as pink noise, which has a substantially constant energy content per octave) is applied to the equalizer input in place of the normal source signal. The frequency characteristics of the acoustic signal within the listening environment is then determined by means of a special microphone and spectrum analyzer. Any difference between the known frequency characteristics of the applied signal and the measured frequency characteristics of the acoustic signal are removed by adjusting the attenuators for the narrow band filters. After adjustement of the equalizer, the test signal source is disconnected from the system and the normal audio signal source is applied through the equalizer to the loudspeaker system.
Since a special set up procedure is required to adjust the equalizer, the equalizer settings are not normally changed very often, certainly not during the normal operation of the system. Consequently, if any changes occur within the listening environment which change its frequency response characteristics, then the fidelity of reproduction will be adversely affected. This may occur, for example, where the user changes the orientation of the speakers, where furniture is moved about in a room, where rugs are added or removed, where windows are opened and closed, where the number of people in the room changes, etc.
Moreover, since the equalizer is of course adjusted with the microphone in a single location, there is no assurance that the response characteristics at other locations will be similarly equalized. Thus, should the user decide to position himself in a different location he must either readjust the equalizer with the microphone in the new location, or tolerate any non-uniformities in the overall frequency response characteristics, as measured at that location.